Chapter 10: File-System Interfacepeople.uncw.edu/ricanekk/teaching/spring09/csc342/slides/ch10... · Silberschatz, Galvin and Gagne ©2009 Single-Level Directory A single directory

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Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8th Edition,

Chapter 10: File-System Interface

10.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8th Edition

Chapter 10: File-System Interface

  File Concept   Access Methods   Directory Structure   File-System Mounting   File Sharing   Protection


Objectives

  To explain the function of file systems   To describe the interfaces to file systems   To discuss file-system design tradeoffs, including access methods, file

sharing, file locking, and directory structures   To explore file-system protection


File Concept

  Contiguous logical address space

  Types:   Data

 numeric  character  binary

  Program


File Structure

  None - sequence of words, bytes   Simple record structure

  Lines   Fixed length   Variable length

  Complex Structures   Formatted document   Relocatable load file

  Can simulate last two with first method by inserting appropriate control characters

  Who decides:   Operating system   Program


File Attributes

  Name – only information kept in human-readable form   Identifier – unique tag (number) identifies file within file system   Type – needed for systems that support different types   Location – pointer to file location on device   Size – current file size   Protection – controls who can do reading, writing, executing   Time, date, and user identification – data for protection, security, and

usage monitoring   Information about files are kept in the directory structure, which is

maintained on the disk

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File Operations

  File is an abstract data type   Create   Write   Read   Reposition within file   Delete   Truncate   Open(Fi) – search the directory structure on disk for entry Fi, and move the

content of entry to memory   Close (Fi) – move the content of entry Fi in memory to directory structure on

disk


Open Files

  Several pieces of data are needed to manage open files:   File pointer: pointer to last read/write location, per process that has the

file open   File-open count: counter of number of times a file is open – to allow

removal of data from open-file table when last processes closes it   Disk location of the file: cache of data access information   Access rights: per-process access mode information


Open File Locking

  Provided by some operating systems and file systems   Mediates access to a file   Mandatory or advisory:

  Mandatory – access is denied depending on locks held and requested   Advisory – processes can find status of locks and decide what to do


File Locking Example – Java API import java.io.*; import java.nio.channels.*; public class LockingExample {

public static final boolean EXCLUSIVE = false; public static final boolean SHARED = true; public static void main(String arsg[]) throws IOException { FileLock sharedLock = null; FileLock exclusiveLock = null; try { RandomAccessFile raf = new RandomAccessFile("file.txt", "rw"); // get the channel for the file FileChannel ch = raf.getChannel(); // this locks the first half of the file - exclusive exclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE); /** Now modify the data . . . */ // release the lock exclusiveLock.release();


File Locking Example – Java API (cont)

// this locks the second half of the file - shared sharedLock = ch.lock(raf.length()/2+1, raf.length(),

SHARED); /** Now read the data . . . */ // release the lock sharedLock.release(); } catch (java.io.IOException ioe) { System.err.println(ioe); }finally { if (exclusiveLock != null) exclusiveLock.release(); if (sharedLock != null) sharedLock.release(); } }

}


File Types – Name, Extension

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Access Methods

  Sequential Access read next write next reset no read after last write (rewrite)

  Direct Access read n write n position to n read next write next rewrite n n = relative block number


Sequential-access File


Simulation of Sequential Access on Direct-access File


Example of Index and Relative Files


Directory Structure

  A collection of nodes containing information about all files

F 1 F 2 F 3 F 4

F n

Directory

Files

Both the directory structure and the files reside on disk Backups of these two structures are kept on tapes


Disk Structure

  Disk can be subdivided into partitions   Disks or partitions can be RAID protected against failure   Disk or partition can be used raw – without a file system, or formatted with a

file system   Partitions also known as minidisks, slices   Entity containing file system known as a volume   Each volume containing file system also tracks that file systemʼs info in

device directory or volume table of contents   As well as general-purpose file systems there are many special-purpose file

systems, frequently all within the same operating system or computer

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A Typical File-system Organization


Operations Performed on Directory

  Search for a file   Create a file   Delete a file   List a directory   Rename a file   Traverse the file system


Organize the Directory (Logically) to Obtain

  Efficiency – locating a file quickly   Naming – convenient to users

  Two users can have same name for different files   The same file can have several different names

  Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)


Single-Level Directory

  A single directory for all users

Naming problem

Grouping problem


Two-Level Directory

  Separate directory for each user

  Path name   Can have the same file name for different user   Efficient searching   No grouping capability


Tree-Structured Directories

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Tree-Structured Directories (Cont)

  Efficient searching

  Grouping Capability

  Current directory (working directory)   cd /spell/mail/prog   type list


Tree-Structured Directories (Cont)

  Absolute or relative path name   Creating a new file is done in current directory   Delete a file rm <file-name>

  Creating a new subdirectory is done in current directory mkdir <dir-name>

Example: if in current directory /mail mkdir count

mail

prog copy prt exp count

Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”


Acyclic-Graph Directories

  Have shared subdirectories and files


Acyclic-Graph Directories (Cont.)

  Two different names (aliasing)

  If dict deletes list ⇒ dangling pointer Solutions:   Backpointers, so we can delete all pointers

Variable size records a problem   Backpointers using a daisy chain organization   Entry-hold-count solution

  New directory entry type   Link – another name (pointer) to an existing file   Resolve the link – follow pointer to locate the file


General Graph Directory


General Graph Directory (Cont.)

  How do we guarantee no cycles?   Allow only links to file not subdirectories   Garbage collection   Every time a new link is added use a cycle detection

algorithm to determine whether it is OK

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File System Mounting

  A file system must be mounted before it can be accessed   A unmounted file system (i.e. Fig. 11-11(b)) is mounted at a

mount point


(a) Existing. (b) Unmounted Partition


Mount Point


File Sharing

  Sharing of files on multi-user systems is desirable

  Sharing may be done through a protection scheme

  On distributed systems, files may be shared across a network

  Network File System (NFS) is a common distributed file-sharing method


File Sharing – Multiple Users

  User IDs identify users, allowing permissions and protections to be per-user

  Group IDs allow users to be in groups, permitting group access rights


File Sharing – Remote File Systems

  Uses networking to allow file system access between systems   Manually via programs like FTP   Automatically, seamlessly using distributed file systems   Semi automatically via the world wide web

  Client-server model allows clients to mount remote file systems from servers   Server can serve multiple clients   Client and user-on-client identification is insecure or

complicated   NFS is standard UNIX client-server file sharing protocol   CIFS is standard Windows protocol   Standard operating system file calls are translated into remote

calls   Distributed Information Systems (distributed naming services) such

as LDAP, DNS, NIS, Active Directory implement unified access to information needed for remote computing

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File Sharing – Failure Modes

  Remote file systems add new failure modes, due to network failure, server failure

  Recovery from failure can involve state information about status of each remote request

  Stateless protocols such as NFS include all information in each request, allowing easy recovery but less security


File Sharing – Consistency Semantics

  Consistency semantics specify how multiple users are to access a shared file simultaneously   Similar to Ch 7 process synchronization algorithms

 Tend to be less complex due to disk I/O and network latency (for remote file systems

  Andrew File System (AFS) implemented complex remote file sharing semantics

  Unix file system (UFS) implements:  Writes to an open file visible immediately to other users of the same

open file  Sharing file pointer to allow multiple users to read and write

concurrently   AFS has session semantics

 Writes only visible to sessions starting after the file is closed


Protection

  File owner/creator should be able to control:   what can be done   by whom

  Types of access   Read   Write   Execute   Append   Delete   List


Access Lists and Groups

  Mode of access: read, write, execute   Three classes of users

RWX a) owner access 7 ⇒ 1 1 1

RWX b) group access 6 ⇒ 1 1 0 RWX c) public access 1 ⇒ 0 0 1

  Ask manager to create a group (unique name), say G, and add some users to the group.

  For a particular file (say game) or subdirectory, define an appropriate access.

owner group public

chmod 761 game

Attach a group to a file chgrp G game


Windows XP Access-control List Management


A Sample UNIX Directory Listing

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Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8th Edition,

End of Chapter 10

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Chapter 10: File-System Interfacepeople.uncw.edu/ricanekk/teaching/spring09/csc342/slides/ch10... · Silberschatz, Galvin and Gagne ©2009 Single-Level Directory A single directory